The present invention pertains to a borehole tool and more particularly to a magnetic compass device which is capable of directional surveys of a borehole having angular changes up to 180.degree. in a vertical direction.
When making drill holes in the ground, particularly drill holes which run partly through rock and partly through less consolidated earth layers, it often happens that the dip or inclination of the drill hole in the horizontal plane as well as the direction of the hole will vary quite considerably at different depths of the hole. In many cases, therefore it is very important to determine the existing deviations from a desired inclination and direction and in addition it is often desirable to survey the complete traverse of an uncased hole. One such situation occurs when it is desirable to determine the true vertical depth of a hole in order, for example, to locate a geological formation relative to the well depth. A complete survey of the borehole traverse is also desirable for the purpose of a "check survey," used to determine if a directionally drilled hole is located at its intended position. However, the most common use for the so-called "multishot" directional survey is to determine the exact bottom hole location or the path of the wellbore. A survey of the path may be used to pick out an optimum point in the wellbore to start the deflection of a directionally drilled hole. Defining the bottom hole location would have applications to many situations as, for example, to determine where the hole bottom is located relative to a stratigraphic feature of a formation, or even for legal purposes.
One apparatus which has been used for the purposes outlined above is termed a multishot magnetic directional survey instrument. Such an instrument is used to obtain a running record of the inclination and the direction of inclination at various depths in a borehole. The instrument may be used for the purpose of orienting a hole in directional drilling operations, and also to chart the course of boreholes from surface to total depth during a normal drilling operation. The directional features of the instrument consist of a magnetic compass. The inclination unit is a form of inverted plumbob. These two features are combined into a single compass angle unit which may be available in various ranges of degrees of inclination. The compass angle unit is normally comprised of a floating magnetic needle designed so that its directional and angular positions may be simultaneously photographed and recorded on a film strip. A movie camera unit in the instrument makes a permanent record of the compass angle unit reading as it traverses the borehole. Electrical power to operate lamps in the camera unit may be furnished by batteries or by a conductor cable suspending the instrument from the earth's surface. Further details of a multishot survey instrument are found in U.S. Pat. No. 3,588,908.
One application for borehole surveys which is particularly pertinent to the present invention is that of mining boreholes. In uranium mining operations, small diameter boreholes, or "fan" holes, are made into earth formations into which are run gama probes. After shafts are sunk and mining begins, a certain amount of exploration is conducted both to determine the extent of ore present and also to find hidden ore bodies. Typically, these holes are drilled from an underground cave into its walls, overhead, and back to horizontal on the opposite side. One "fan" consists of approximately twelve holes. There is no control used in the drilling process to reach an objective target. Instead holes are allowed to drift at will. The holes are dug from between 200 ft. to 500 ft. deep.
After a fan is completed, logging operations are used to probe the holes, gathering a log of gamma count correlated to its depth. Because of the random wandering of the borehole and the expensive cost of drilling the ore (estimated at $1,000 per foot), surveying has become a necessity for accurate location of ore bodies.
The present methods used in logging and surveying fan holes are as follows: Crews of two men begin on a series of fan holes by inserting a logging probe using five foot sections of aluminum push rods. The hole is logged by the probe as rods are pulled from the bore. A conductor line is used for transmission of data from the probe to a display readout at the operator's console. A self-contained 24-volt power supply is used to run the probe. Using the presently available battery powered multishot survey instrument and circuit breakers during the gamma logging process causes the operator to have to wait for a minimum of 35 seconds at a survey station to obtain a record. The battery powered multishot continuously records the position of the compass on film at intervals selected by a switch on one end of the tool. Intervals are available from 15 seconds to 32 minutes. To insure a good record at a particular depth, it is first necessary to allow the compass to stabilize. This takes approximately 10 seconds. Then to insure a photograph is taken at the location after the compass is stabilized, the operator must wait 15 seconds for the lamps to start to be activiated. After the lamps are activated, it takes approximately 10 seconds to obtain an adequate exposure of the film. This waiting time delays the total process sufficiently that in surveying uranium mines, the survey itself has been separated from the logging process and run alone subsequent to the logging of a fan. The total process of two runs, one for logging and one for surveying, has presented significant problems in the amount of personnel required and effectiveness of the survey.
Needed is a survey instrument which can be activated from the surface thereby eliminating the waiting time required in obtaining a survey record. Thus the log and directional survey can be done on one trip in the hole, and total time cut in half.
One solution to the above-mentioned problem is a system having the instruments, lights, and solenoid advance powered from the surface using conductor wires. This method has proven only partially successful. By using a compass capable of tilting up to 120.degree. from vertical, holes surveyed in this fashion cannot exceed 20.degree. above horizontal (120.degree. from vertical). When using a standard battery powered multishot, the instrument can be inverted in its protective case eliminating this problem. However, when using a surface-activated multishot, the instrument requires a conductor wire running along side the instrument when it is inverted, and this wire creates a magnetic field affecting the compass reading when current is applied to power the lights.
Some of the more obvious solutions which have been advanced to solve this problem are as follows:
(a) To use some type of shielding device around the compass itself preventing magnetic interference. This would necessitate a change in the compass shell or an additional magnetic barrier which cannot be incorporated because of size limitations, particularly tool diameter, due to the small diameter (13/8") of a "fan" hole. PA1 (b) The possibility of building a 180.degree. compass unit. Because of design and engineering problems involved, this method also appears to be impractical. PA1 (c) The use of a time delay circuit which would allow the current passing by the compass to be stored in a series of capacitors at the solenoid end of the instrument and then released to power the lights at a predetermined delay period after the compass has come to rest from its electrical current interference. Again, such a system would get into space, electronic complexity, and time delay problems.
It is therefore an object of the present invention to provide a simple system meeting the physical space requirements of a small diameter tool housing for directionally surveying holes up to 180.degree. from vertical and being surface activated.